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Re: Another DIY SS Amplifier with NVA Boards
Posted: Mon Jan 03, 2022 2:23 pm
by savvypaul
SteveTheShadow wrote: ↑Sun Jan 02, 2022 11:41 pm
This integrated DIYNVA amp concept seems to be extremely successful. If Paul and Tomasz are resurrecting the idea of an integrated, then good on them. Offering the future NVA integrated with an upgrade of a built-in moving magnet only phono stage, at a price premium, whilst keeping MC capability and multiple regulators exclusively with the separate phonos, could be something to think about. Integrating the MM phono board into my own setup seems to have produced no problems, only benefits. In a commercial product however, it might be out of the question.
Under consideration, but not straightforward, and it has implications for the layout of the amp (phono stage fitted or not) as we would want it to be retrofittable.
Re: Another DIY SS Amplifier with NVA Boards
Posted: Mon Jan 03, 2022 2:51 pm
by Latteman
Is taking the power + & - from the amp board supply a ‘common’ method in integrated amps- I wonder if there is any effect on the power delivery, current or voltage to the input stage of the amp boards. Is there sufficient in reserve with the capacitors used?
Great looking amp and build
Re: Another DIY SS Amplifier with NVA Boards
Posted: Mon Jan 03, 2022 2:54 pm
by savvypaul
Latteman wrote: ↑Mon Jan 03, 2022 2:51 pm
Is taking the power + & - from the amp board supply a ‘common’ method in integrated amps- I wonder if there is any effect on the power delivery, current or voltage to the input stage of the amp boards. Is there sufficient in reserve with the capacitors used?
Great looking amp and build
We would consider doing it in an amp fitted with separate input and output transformers, taking the power for the phono from the input transformer.
Re: Another DIY SS Amplifier with NVA Boards
Posted: Mon Jan 03, 2022 4:28 pm
by SteveTheShadow
Latteman wrote: ↑Mon Jan 03, 2022 2:51 pm
Is taking the power + & - from the amp board supply a ‘common’ method in integrated amps- I wonder if there is any effect on the power delivery, current or voltage to the input stage of the amp boards. Is there sufficient in reserve with the capacitors used?
Great looking amp and build
Just to add to Paul’s comment. In the days when integrated amps had phono stages, there was no problem with powering them from the single power transformer. My old Naim NAIT 2 would be a good example, or any A&R amp such as the A60 I once had: any standard amp for that matter. Phono boards don’t tend to draw much current, and they would have been decoupled from the other stages by the use of RC filters.
Mine is of course decoupled by a regulator, as would be any phono stage built into an NVA integrated amp.
Re: Another DIY SS Amplifier with NVA Boards
Posted: Fri Jan 07, 2022 11:02 pm
by r3xj0hn570n
SteveTheShadow wrote: ↑Fri Dec 31, 2021 8:24 pm
I also took the opportunity to attenuate the input with a potential divider ahead of the volume control. With this, the 2V output of the CD player and streamer drops to 800mV and with my high efficiency speakers, I can now get the volume above eight o’clock without blowing out the windows.
An alternative to simply attenuating the input is increasing the feedback ratio of the amp. This has other advantages too.
The NVA circuit normally has about 30dB of gain. You can drop 3dB out of this by dropping the series 18k resistor to 10k. It's this one.
- steves_amp2.jpeg (149.68 KiB) Viewed 2882 times
Re: Another DIY SS Amplifier with NVA Boards
Posted: Sat Jan 08, 2022 9:35 am
by SteveTheShadow
r3xj0hn570n wrote: ↑Fri Jan 07, 2022 11:02 pm
An alternative to simply attenuating the input is increasing the feedback ratio of the amp. This has other advantages too.
The NVA circuit normally has about 30dB of gain. You can drop 3dB out of this by dropping the series 18k resistor to 10k. It's this one.
Thanks. I’ll have a look at that option and thanks for pointing me to the location of said resistor. I’m a bit concerned that increasing the feedback levels might introduce instability, though I don’t know an awful lot about transistor circuitry. With that in mind, I could of course be worrying unnecessarily.
Re: Another DIY SS Amplifier with NVA Boards
Posted: Sat Jan 08, 2022 9:36 pm
by karatestu
SteveTheShadow wrote: ↑Sat Jan 08, 2022 9:35 am
r3xj0hn570n wrote: ↑Fri Jan 07, 2022 11:02 pm
An alternative to simply attenuating the input is increasing the feedback ratio of the amp. This has other advantages too.
The NVA circuit normally has about 30dB of gain. You can drop 3dB out of this by dropping the series 18k resistor to 10k. It's this one.
Thanks. I’ll have a look at that option and thanks for pointing me to the location of said resistor. I’m a bit concerned that increasing the feedback levels might introduce instability, though I don’t know an awful lot about transistor circuitry. With that in mind, I could of course be worrying unnecessarily.
How about we let Rex try it out first and then he can report back on any fireworks
Re: Another DIY SS Amplifier with NVA Boards
Posted: Sat Jan 08, 2022 9:58 pm
by r3xj0hn570n
karatestu wrote: ↑Sat Jan 08, 2022 9:36 pm
How about we let Rex try it out first and then he can report back on any fireworks :grin:
Fair enough. Next time i have my test amp apart i'll try it. Hmm , could just jumper a 22k on top...
There are other circuit mods i won't be reversing, but they don't affect the circuit stability.
Re: Another DIY SS Amplifier with NVA Boards
Posted: Thu Jan 13, 2022 4:53 am
by r3xj0hn570n
OK, here is a rebuilt original curcuit AP20, -10dBFS 1kHz sine out the sound card.
- AP20-original-at--10dBFS.jpg (155.19 KiB) Viewed 2676 times
Here it is with the 18k feedback resistor jumpered with a 22k.
- AP20-10krfb-at--10dBFS.jpg (148.79 KiB) Viewed 2676 times
Less output as predicted. Nothing untoward showing on my scope.
As an addendum, here is the output with the differential input emitter resistors (120R) jumpered (and FB back to 18k)
- AP20-diff-bypass-at--10dBFS.jpg (152.06 KiB) Viewed 2676 times
Re: Another DIY SS Amplifier with NVA Boards
Posted: Thu Jan 13, 2022 7:38 am
by karatestu
r3xj0hn570n wrote: ↑Thu Jan 13, 2022 4:53 am
OK, here is a rebuilt original curcuit AP20, -10dBFS 1kHz sine out the sound card.
AP20-original-at--10dBFS.jpg
Here it is with the 18k feedback resistor jumpered with a 22k.
AP20-10krfb-at--10dBFS.jpg
Less output as predicted. Nothing untoward showing on my scope.
As an addendum, here is the output with the differential input emitter resistors (120R) jumpered (and FB back to 18k)
AP20-diff-bypass-at--10dBFS.jpg
Thanks for that Rex, that is very helpful.
